Increases in fuel prices as well as in the regulation of exhaust gas emissions have resulted in an increase in the popularity of more fuel efficient propulsion systems for automobiles. High efficiency internal combustion engines, diesel engines and hybrid propulsion systems, that utilize a propulsion combination of an electric motor and an internal combustion engine, are just some of the solutions that vehicle designers are considering to solve the efficiency and regulatory challenges. With conventional, non-hybrid powertrains, about 30 percent of the fuel energy is lost to the exhaust system in the form of waste heat. With hybrid propulsion systems, the maintenance of appropriate temperatures in the engine, the transmission, the exhaust system and in the passenger compartment during cold ambient temperature operation may be a challenge due to frequent starting and stopping of the internal combustion engine.
In either case, the engine exhaust system with its large temperature differential vis-à-vis the ambient, for instance, is a convenient target for recovering useful energy in the form of waste heat for use elsewhere in the vehicle. A heat exchanger may be associated with the engine exhaust system and may, for instance, use engine coolant, or another working fluid, to extract waste heat from the exhaust flow through the exhaust system. Heat recovered by a heat exchanger may be used to assist in rapidly heating the engine or transmission following a cold start to reduce friction and increase system efficiency. Alternatively, the recovered heat may be used to generate electricity through thermo-electric power generation in which a temperature differential creates a voltage which may be used by the vehicle's electrical system. Because exhaust temperatures may vary widely across operating cycles (from about 300° C. to about 1000° C.) the various vehicle systems that utilize the recovered exhaust heat, including the heat exchanger itself, must be monitored for over-temperature conditions in which the recovered heat or energy can be detrimental. While the exhaust temperature is not easily controlled, in such instances it is typical to bypass the engine exhaust gas around the heat exchanger for release to the atmosphere after appropriate treatment in various exhaust treatment devices. Bypass of the exhaust gas around the heat exchanger can require the use of a bypass valve mounted in fluid communication with the hot exhaust gas. In such an environment, the valve is subjected to extreme harshness requiring very robust and often expensive designs and materials.